1. Field of the Invention
This invention relates generally to plasma processing applications utilizing RF power and more particularly to RF power generators used in plasma processing applications having circuitry to improve power delivery characteristics.
2. Brief Description of the Prior Art
Heretofore, RF power generators used in plasma applications have relied on relatively simple diode peak detectors operating from the output of a directional coupler to monitor the power delivered to the plasma However, this method becomes unreliable in the presence of plasma induced spurious frequencies because the diode detectors cannot differentiate between the voltage of the generator output frequency and that of the plasma induced spurious frequencies. Synchronous detection methods have been used to eliminate the effect of spurious frequencies, however the cost and complexity of such a design is generally not acceptable for plasma generator applications. Other power detectors relying on thermal response have also been used. Bandpass filters of various types have been used to eliminate spurious frequencies. However, insertion losses in these filters are difficult to control, particularly when the offending frequency is very close to the generator output frequency.
FIG. 1 shows a typical VHF power generator 10 of the prior art. A rf frequency source 12 supplies power to an amplifier 14 which is fed to a directional coupler 16. The directional coupler 16 is connected to diode detectors 18 and 20. In some designs filters (not shown) are used between the directional coupler 16 and the detectors 18 and 20 to remove spurious frequencies. However, design of these filters is very difficult when the spurious frequencies are very close to the generator output frequency. For example, if the spurious frequency has an amplitude that is higher than the generator output and this signal is close in phase to the output frequency of the generator that is being fed back to monitor the power delivered, the peak diode detectors cannot distinguish between the true feedback signal and the signal of the spurious frequency. Therefore, the detected signals connected to the feedback power control circuitry 22 would be in error. A matching network 24 is connected between the generator 10 and plasma chamber 26 in a typical system application. A second generator 30 coupled through matching network 32 may supply power to the chamber at a different operating frequency.
It would be desirable if there were provided an RF generator that had improved power control stability by isolating the feedback control voltage from the plasma induced spurious frequencies.
Accordingly, it is an object of this invention to provide an improved RF power control method that is easily implemented to replace more expensive and complex designs currently used in the prior art.
It is a further object of this invention to replace the complex narrowband filters as used in prior art with a relatively simple low pass filter to remove the spurious signals.
It is yet another object of this invention to provide a means for using methods other than voltage detection of the generator output for monitoring its power.
It is yet another object of this invention to provide a means for improved matching network tuning in the presence of spurious frequencies.
There is provided by this invention an improved RF power control method for plasma applications that optimizes the feedback control voltage in the presence of harmonic and non-harmonic spurious caused by interaction between multiple generators acting on the non-linear plasma. In this system, an oscillator and mixer are placed at the sampled output of the solid state RF source used for plasma ignition. The sampled output is mixed to an intermediate frequency and filtered to remove the spurious frequencies that are created in the non-linear plasma. In this way, the feedback power control essentially ignores the spurious frequencies. In this application, the oscillator and mixer do not interfere with other desirable system characteristics and effectively isolate the feedback control voltage from changes in plasma spurious frequency content. This allows RF power to be delivered to the plasma with greater accuracy than would otherwise be possible with conventional power control methods